Various components and devices are employed in conventional exhaust gas aftertreatment systems to reduce the emission of harmful pollutants. One specific component that is often included in exhaust gas aftertreatment systems is a particulate matter filter. Particulate matter filters trap pollutants, such as unburned hydrocarbons, soot, and other particulates. As particulates accumulate on the filter, the flow path of exhaust gas through the filter becomes more restrictive, thus increasing the pressure drop across the filter and increasing backpressure on the engine. In order to determine when a particulate filter requires cleaning (also known as regeneration or reactivation), conventional engine systems often monitor the pressure drop across the particulate filter and initiate a regeneration procedure when the pressure reaches a certain level.
However, if a particulate filter in an exhaust stream has cracks or leaks, the pressure measurement will not accurately reflect the level of particulate loading on the filter. Further, as emissions standards become more stringent, pressure monitoring control systems are generally inadequate in accurately determining the level and degree of particulate accumulation on a filter. Conventional solutions to this problem include implementing a particulate matter sensor in the exhaust gas flow. Particulate matter sensors, used to measure soot and other particulate matter, typically consist of a non-conductive substrate, most often alumina, with a screen-print pattern of conductive material. The conductive material, which may be a precious metal in order to withstand the temperature of exhaust gas streams, functions as a sensing element to determine the level of particulate accumulation. In some instances the particulate matter sensor will also have a heating element to heat the soot sensor during a sensor regeneration procedure.
The sensing element (conductive material) of the particulate matter sensor generally has two electrodes with inter-digitized ‘fingers’ that maximize a perimeter between the two electrodes. When soot from the exhaust gas is deposited onto the sensor, the carbon component of the soot creates a high resistance short between the electrodes, which effectively lowers the resistance. The more soot that accumulates on the sensing element, the lower the resistance. Once a predetermined amount of soot is on the sensing element, it is often desirable to clean off the soot from the sensor. Accordingly, the heating element on the particulate matter sensor may be activated to oxidize the soot and regenerate the sensor. The temperature of the heating element is often an important factor in the sensor regeneration process. If the regeneration temperature is too low, then not enough soot will be removed from the sensor or the sensor regeneration procedure will take an excessive amount of time. If the temperature is too high, the various components of the sensor may be damaged due to the high temperatures. While conventional solutions involve implementing a temperature sensor to monitor the temperature of the heating element, the addition of temperature sensors adds cost and complexity to the aftertreatment system.